Thermoelectric devices convert waste heat into electricity the conversion efficiency of which depends on the thermoelectric material's figure-of-merit. The thermoelectric figure of merit (ZT) is given by, ZT=S2σT/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, κ the thermal conductivity and T is the temperature.
Most of the currently available thermoelectric materials have lower figure-of-merit leading to low conversion efficiency of the thermoelectric device and thus these materials have limited commercial applications. The highest value of thermoelectric figure of merit ˜2.2 reported thus far is for Lead-Silver-Antimony-Tellurium (LAST) alloy. However, LAST alloy contains Lead which is very toxic, Silver and Tellurium which are quite expensive. In contrast, the thermoelectric material nanostructured Cu2Se, described in this invention, is relatively cheap and non-toxic material with a high thermoelectric figure of merit of 2.
Cu2Se is a known thermoelectric material in the literature, and has been synthesized by three different research groups.
Reference may be made to Journal Xiao Xing-Xing et. al. (Chin. Phys. B, vol. 20 (2011) pp. 087201, wherein the synthesis of Cu2Se was carried out by melting high purity Copper and Selenium powders in the desired ratios and sealed in a quartz tubes under vacuum and the tubes were heated up to 1403 K at a heating rate of 2K/min and held at this temperature for another 10 hours, then quenched on cold salt water. The obtained ingot were pulverized into powder and then sintered by a spark plasma sintering technique at 973 K under a pressure of 35 MPa for 7 minutes. The resulting material exhibited a highest ZT of 0.38 at 750 K.
Reference may be made to Journal Huili Liu et al. (Nature Materials, vol. 11 (2012) pp. 422-425), wherein the Cu2Se Polycrystalline samples were prepared by melting the 99.999% pure Cu and Se elements in a pyrolitic boron nitride crucible enclosed in a fused silica tube at 1,423K for 12 h in vacuum, and then slowly cooled down to 1,073K in 24 h and held there for seven days. Finally, the tubes were furnace cooled to room temperature. The resulting ingots were ground into a fine powder by hand using an agate jar and plunger and subjected to spark plasma sintering around 710K under a pressure of 65 MPa. The resulting Cu2Se materials exhibited a ZT of 1.5 at 1000 K.
Reference may be made to Journal Bo Yu et. al (Nano Energy, vol. 1 (2012, pp. 472-478) wherein Cu2Se nanopowders were synthesized from Cu (99.5% pure), and Se (99.99% pure) elements through high-energy ball milling. Bulk samples were fabricated by consolidating the as-prepared nanopowders in a graphite die using a conventional hot pressing method.
In the above references of Xiao Xing-Xing et. al (Chin. Phys. B, vol. 20 (2011) pp. 087201 and Huili Liu et al. (Nature Materials, vol. 11 (2012) pp. 422-425), Cu2Se bulk material was prepared by melting route, wherein in the present invention we have synthesized nanostructured Cu2Se. Although Bo Yu et al. (Nano Energy, vol. 1 (2012, pp. 472-478) have prepared nanostructured Cu2Se by ball milling, but they have sintered these nanopowders by hot pressing route, which is known to result in grain growth. On the contrary, in this invention we have prepared the Cu2Se nanopowders by ball milling, which is then followed by the spark plasma sintering, which has the advantage of fast sintering, producing products with very high density and is known to retain the nanostructure in Cu2Se, leading a to high value 2 for ZT. This value of ZT of 2 in the present invention is the highest reported so far in the literature for nanostructured copper-selenide.